The present application claims priority to currently pending United Kingdom Patent Application number 0212727.2, filed on May 31, 2002.
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Frequency modulation (FM) and phase modulation (PM) techniques are used for transmitting information on a carrier signal, e.g. sound information on a carrier radio wave. Unlike amplitude modulation techniques (AM) where the amplitude of a carrier signal is modulated by the information signal, in FM and PM, the frequency and phase of the carrier signal, respectively, is modulated. FM and PM are usually referred to collectively as xe2x80x98angle modulationxe2x80x99 techniques. In this description, although the term frequency modulation, or FM, is used throughout, it should be understood that the term is intended to apply equally to PM techniques.
As will be appreciated by those skilled in the art, a quality measure for FM modulation systems is the xe2x80x98modulation indexxe2x80x99, which is defined as the ratio of the frequency deviation during modulation to the modulating frequency. Certain applications require a high modulation index, whilst others require a low modulation index. In applications where a high modulation index accuracy is desirable, such as in a radio system, a number of circuit arrangements can be used.
Referring to FIG. 1, a known FM circuit employs the well known I (in-phase) and Q (quadrature) modulation scheme. First and second modulating inputs 3, 5 are provided for receiving an input signal in phase quadrature. The signals are applied to first and second balanced modulators 7, 9 that receive, on first and second lines 10, 11 a quadrature input from a quadrature phase shift network 13. The quadrature input is received at the signal frequency in accordance with the operation of a carrier frequency source 15. The outputs from the first and second modulators 7, 9 are passed, on respective lines 17, 19, to a summing circuit 21. The summed output is provided on output line 23.
Modulation circuits such as that shown in FIG. 1 are used widely and are able to produce almost any form of modulation, albeit with some limitations in frequency shift keying (FSK) modulation. However, the provision of the two balanced modulators 7, 9 with accurate balance of gain and phase, and the provision of the phase shift network 13, results in a complex circuit that requires relatively high power.
A simplified system is shown in FIG. 2. Here, a voltage-controlled oscillator (VCO) 27 feeds a frequency divider 29, usually implemented as a digital counter. The output from the divider 29 provides one input to a phase detector 31, the other input being taken from a reference frequency source 33. The phase detector 31 outputs an error signal representing the phase difference between the two input signals. This error signal is fed to a low pass filter 35 and then to a summing circuit 37 that is fed back to a control input of the VCO 27. A modulation signal is applied from a supply means 39 to a further input of the summing circuit 37. The VCO 27 is directly modulated by the summation result, and the modulated output is taken as the output from the VCO. The loop bandwidth of the circuit of FIG. 2 is determined by a number of factors, and the modulation frequency or frequencies may be totally inside or outside of the bandwidth, or even partially inside and outside the bandwidth.
Systems such as that shown in FIG. 2 have been in use for many years. They offer the advantage that complexity is minimized, and the component count is reduced so that power consumption can be kept at a minimum. The general layout of the circuit components of FIG. 2 will be recognized as being similar to a phase locked loop (PLL) and the bandwidth of the loop is chosen such that the modulation frequency lies outside the bandwidth. In many systems this is because the channel spacing requirements are such that in order to achieve adequate suppression of reference frequency components at the VCO, it is impractical if the attenuation curve of the low pass filter does not start at a low value. A general problem with such systems is that the frequency deviation needs to be set for each different modulation system on an individual basis because of the difficulties in producing a VCO in which the control voltage coefficient Kv (controlling the frequency change in Hertz per volt) is constant within a single production batch. Further problems appear since the value of Kv varies with supply voltage and/or temperature.
The invention, in one sense, relates to a system for performing frequency modulation which includes a VCO and a phase detector configured to receive an output signal from the VCO, the phase detector being arranged to output an error signal representing the phase difference between the signal output from the VCO and a reference signal, the system also including control means arranged to monitor the error signal to derive an indication of the frequency deviation and, as a result of the derivation, to maintain the frequency deviation of the VCO substantially constant. In the context of the application, xe2x80x98VCOxe2x80x99 is intended to mean any circuit or system whereby the frequency of an output voltage from the circuit or system is dependent on a voltage (often referred to as a xe2x80x98control voltagexe2x80x99) inputted therein.
According to a further aspect of the present invention, there is provided a frequency modulation system comprising: a VCO; means for summing a modulating input signal and an error signal thereby to generate a signal for controlling the frequency of a signal outputted from the VCO; a phase detector for generating the error signal based on the detected phase difference between the signal outputted from the VCO and a reference signal; and control means arranged (a) to receive the error signal and to derive an indication of the frequency deviation of the VCO; and (b) to vary the output from the summing means in accordance with the frequency deviation thereby to maintain the frequency deviation of the VCO substantially constant.
The system may further include a controllable gain module arranged to output, to the summing means, an amplified version of the modulating input signal, the control means being arranged to vary the output from the summing means by controlling the amount of amplification applied to the modulating input signal.
The control means may be arranged to generate, from the error signal, a signal representative of the frequency deviation of the VCO, and to generate a gain signal for controlling the gain applied by the controllable gain module.
The control means may include means arranged to determine the amplitude of the error signal, and comparator means arranged to compare the determined amplitude with a predetermined reference voltage, the output from the comparator means comprising the gain signal for controlling the gain of the controllable gain module.
The amplitude determining means comprises a rectifier. In digital modulation applications, the amplitude determining means may include a sample and hold circuit. The system may further comprise a frequency divider connected between the VCO and the phase detector.
According to a further aspect of the invention, there is provided a frequency modulation system comprising: a VCO; means for summing a modulating input signal and an error signal thereby to generate a signal for controlling the frequency of a signal outputted from the VCO; a phase detector for generating the error signal based on the detected phase difference between the signal outputted from the VCO and a fixed frequency reference signal; and control means arranged (a) to receive the error signal and to derive a controlling signal that is proportional to the frequency deviation of the VCO, and (b) to vary the output from the summing means, using the controlling signal, thereby to maintain the frequency deviation substantially constant.
According to further aspect of the invention, there is provided a method of performing frequency modulation in a system comprising a VCO, means for summing a modulating input signal and an error signal thereby to generate a signal for controlling the frequency of a signal outputted from the VCO, and a phase detector for generating the error signal based on the detected phase difference between the signal outputted from the VCO and a reference signal, wherein the method comprises: monitoring the error signal and deriving an estimate of the frequency deviation exhibited by the VCO; and controlling the output of the summing means in accordance with the estimated frequency deviation such that the frequency deviation of the VCO is maintained substantially constant.
While the above-mentioned FM systems and methods can be used in radio system applications, the same also applies to optical system applications.
Additional objects and advantages of the invention will be set forth in part in the description that follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate at least one presently preferred embodiment of the invention as well as some alternative embodiments. These drawings, together with the description, serve to explain the principles of the invention but by no means are intended to be exhaustive of all of the possible manifestations of the invention.